Pd/Ni-WO3 anodic double layer gasochromic device

ABSTRACT

An anodic double layer gasochromic sensor structure for optical detection of hydrogen in improved response time and with improved optical absorption real time constants, comprising: a glass substrate; a tungsten-doped nickel oxide layer coated on the glass substrate; and a palladium layer coated on the tungsten-doped nickel oxide layer.

This application claims priority from U.S. Provisional Application Serial No. 60/202,501 filed May 5, 2000.

CONTRACTED ORIGIN OF THE INVENTION

The United States Government has rights in this invention under Contract No. DE-AC36-99GO10337 between the United States Department of Energy and the National Renewable Energy Laboratory, a division of the Midwest Research Institute.

TECHNICAL FIELD

The invention relates to a Pd/Ni WO₃ (palladium/tungsten-doped nickel oxide) anodic double layer gasochromic device in which the palladium layer functions as a catalyst material that facilitates reaction with hydrogen gas. The hydrogen gas is disassociated on the Pd catalyst into H atoms, which diffuse into the Ni—WO₃ film. The Ni—WO₃ thin film exhibits an anodic coloration with H⁺ or Li⁺ insertion. The Ni—WO₃ thin film is more stable than WO₃ films in air, due to the fact that Ni oxide based materials, unlike WO₃, forms a hydroxide upon absorption of water vapor. Even after forming the hydroxide, the Ni—W hydroxide thin film still shows a strong color change. By use of the gasochromic response upon exposure to hydrogen gas, hydrogen gas monitoring of the anodic double layer device of the invention can be detected via optical detection schemes such as a fiber-optic type H₂ sensor.

1. Background Art

Hydrogen is a plentiful, clean, non-polluting fuel. Hydrogen is currently used in many industries, and the US demand for hydrogen is approximately 140 billion cubic feet per year and growing. However, hydrogen is explosive at 4% in air. Therefore, it is critical to measure, monitor and control hydrogen wherever it is used.

In the gasochromic art where sensors and measurement instrumentation for hydrogen gases detect and/or measure hydrogen, typically there is required a portable sensing device, a kit (where hydrogen gas detection and/or measurement is required in existing equipment), and sensor heads installed at points where hydrogen leaks are possible, or where monitoring is necessary (i.e., in internal combustion engines which operate using hydrogen as a fuel).

The problems associated with current H₂ gasochromic devices are that these devices are not of adequate durability in that they degrade quickly with cycling and time, are too moisture sensitive, and react too slowly in response to the presence of H₂ to produce an optical absorption change with a lengthy time constant in the vicinity of 30 seconds.

2. Description of the Related Art

At present, optical detection of H₂ is widely accomplished through the use of Pd/WO₃ hydrogen detecting gasochromic devices. However, several problems or drawbacks are associated with the use of Pd/WO₃ hydrogen detecting gasochromic devices. These problems are: they are of inadequate durability; they respond slowly to the presence of H₂; and there is a conflicting cathodic-anodic optical response that results in a weak color change.

Inadequate durability problems are occasioned by the fact that the Pd/WO₃ hydrogen detecting gasochromic device degrades quickly with cycling and time, and is unduly moisture sensitive.

The slow response of the Pd/WO₃ hydrogen detecting gasochromic device in the presence of a H₂ leak is due to the hydrogen reaction in H_(x)WO₃ which produces a slow optical absorption change within a lengthy room temperature time constant of about 30 seconds.

Also, there is a conflicting optical response upon detection of H₂ by the Pd/WO₃ gasochromic device due to the fact that the WO₃ exhibits a cathodic response and the Pd exhibits an opposite anodic response.

3. Disclosure of Invention

One object of the present invention is to provide an anodic double layer H₂ detecting gasochromic device of improved durability that shows little degradation with cycling and time.

A further object of the present invention is to provide an anodic double layer H₂ detecting gasochromic device that responds more swiftly to detection of H₂ gas by producing faster optical absorption change within a room temperature time constant of about 10 seconds.

Another object of the present invention is to provide an anodic double layer H₂ detecting gasochromic device comprising complementary coloring layers in which both of the layers consist of an anodic coloration material.

In general, the invention is accomplished by providing a palladium/tungsten-doped nickel oxide anodic double layer gasochromic device in which, a Ni—WO₃ thin film is prepared on a glass substrate by reactive sputtering. Thereafter, a palladium layer is evaporated onto the Ni—WO₃ thin film. The palladium layer serves as a catalyst material that facilitates reaction with hydrogen gas. That is, the hydrogen gas is dissociated on the Pd catalyst into H atoms, which readily diffuse into the Ni—WO₃ film. The Ni—WO₃ thin film exhibits an anodic coloration with insertion of either H⁺ or Li⁺.

The Ni—WO₃ thin films arc more stable than WO₃ films in air due to the fact that Ni oxide based materials, unlike WO₃, form a hydroxide upon absorption of water vapor. Even after formation of the hydroxide, Ni—W hydroxide thin films still show a strong color change. By use of this gasochromic response upon exposure to hydrogen gas, hydrogen gas can be monitored via optical detection schemes such as fiber-optic type H₂ sensors.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts gasochromism of a Pd/EC oxide H₂ device in which hydrogen gas is disassociated on the Pd catalyst into H atoms.

FIG. 2 is a chart showing percent transmission versus time for cycling results of a prior art Pd/WO₃ hydrogen sensor device.

FIG. 3 is a graph showing percent transmission versus time for cycling results of the PdNi—WO₃ hydrogen sensor of the invention.

FIG. 4 is a diagram showing a fiber-optic H₂ sensor in which the gasochromic response upon exposure to H₂ is measured.

BEST MODE FOR CARRYING OUT THE INVENTION

Due to the fact that Pd/WO₃ sensors are encumbered by inadequate durability, slow response time, and conflicting optical responses upon detecting H₂, a need exists in the interest of safety to provide H₂ sensors of improved durability, faster response time, and a non-conflicting optical response when detecting hydrogen, which is explosive at 4% in air.

The Pd/WO₃ sensor is of inadequate durability because it degrades quickly with cycling and time and is unduly moisture sensitive. Further, the slow response time of Pd/WO₃ sensors is due to hydrogen reaction in H_(x)WO₃ which produces optical absorption change with a room temperature time constant of 30 seconds. Further still, the Pd/WO₃ sensor exhibits a conflicting optical response due to the fact that the WO₃ undergoes a cathodic response and the Pd exhibits an anodic response.

The improved gasochromic device of the invention is obtained by preparing Ni—WO₃ thin film on a glass substrate by reactive sputtering. Thereafter, a palladium layer is evaporated onto the Ni—WO₃ thin film. The palladium layer is used as a catalyst material to facilitate reaction with hydrogen. When the palladium layer reacts with hydrogen, hydrogen gas is dissociated on the Pd catalyst into H atoms, which readily diffuse into the Ni—WO₃ film.

The Ni—WO₃ thin films show an anodic coloration upon insertion of H⁺ or Li⁺, unlike WO₃, as follows:

Ni³⁺Ni²⁺.

The Pd also shows anodic coloration in accordance with the following equation:

PdPdH_(x).

It has been found that the Ni—WO₃ thin films are much more stable than WO₃ in air because the nickel oxide based materials, unlike the WO₃ tend to form a hydroxide with absorption of water vapor, and even after forming the hydroxide, the Ni—W hydroxide thin film still shows a strong color change.

Reference is now made to FIG. 1, which shows the gasochromism of a palladium electrochromic (EC) oxide H₂ device. In this device, hydrogen gas is dissociated on the Pd catalyst into H atoms, and the H atoms diffuse into the WO₃ layer in accordance with the following formula:

xH⁺ +xe ⁻+WO₃=H_(x)WO₃,

where W⁶⁺→W⁵⁺

FIG. 2 is a graph showing percent relative transmission versus time for cycling results of the Pd/WO₃ gasochromic hydrogen sensing device.

FIG.3 is a graph showing percent relative transmission versus time for cycling results of the PdNi—WO₃ anodic double layer hydrogen sensor of the invention.

The PdNi—WO₃ sensor shows little degradation with cycling and time. Further, the hydrogen reaction in the Pd/Ni—WO₃ produces a fast response optical absorption change with a room temperature time constant of just 10 seconds, because the Ni—WO₃ thin film shows anodic coloration with H⁺ insertion (unlike WO₃), per:

Ni³⁺(color)Ni²⁺(transparent)

Because the Pd also shows anodic coloration per PdPdH_(x), there is very strong color change upon detection of H₂ as there is no conflicting cathodic and anodic response as in the case of the Pd/WO₃ gasochromic device. This strong color change is further aided by the fact that, while WO₃ films are sensitive to air (and the moisture therein), the Ni—WO₃ films form a hydroxide with air as follows:

 Ni—W oxide→NiW(OH)₂,

and due to the fact that nickel hydroxide is moisture insensitive, it does not off-set the strong color change of the anodic coloration's due to the nickel oxide and palladium complementary coloring layers.

Reference is now made to FIG. 4 in which a diagram shows a fiber-optic H₂ sensor in which the gasochromic response upon exposure to H₂ is measured The exploded or enlarged sectional view shows the reaction caused by detection of the presence of hydrogen by the palladium catalyst layer and the diffusion of the H⁺ into the WO₃ layer in accordance with the following formula:

4H₂+O₂ +nWO₃ nH_(4/n)WO₃+2H₂O

In FIG. 4, another exploded or sectional view of the meter measurement for hydrogen detection comprises the following key designated components:

C-Coupler, L-LED, D-Dichroic Mirror, S-Signal photodiode, R-Reference photodiode.

The optical absorption in. wavelength based upon the amount of hydrogen is shown in nm.

The invention device applied to an optical fiber comprises placing the Ni—WO₃ thin film coated with the Pd layer onto an optical fiber. The adaption of the invention device to a fiberoptic sensor is important because:

1) It allows elimination of an ignition energy source at the leak site, thereby limiting the risk of an explosion;

2) The fiber-optic signal transmission is immune to electromagnetic interference; and

3) The sensor and opto-electronic are very economical to match-up.

While the invention Pd/Ni—WO₃ device is primarily for purposes of sensing H₂ due to its improved kinetics and durability in air, it is to be emphasized that the invention works equally as well in that it also exhibits anodic coloration upon insertion of Li⁺. Further, the hydrogen sensing oxide material, in addition to being. Ni—WO₃ may be Ni—Ta oxide to obtain reversible coloration≠bleaching. 

What is claimed is:
 1. An anodic double layer gasochromic sensor structure for optical detection of hydrogen in improved response time and with improved optical absorption real time constants, comprising: a substrate; a nickel-tungsten oxide layer coated on said substrate; and a palladium layer coated on said nickel-tungsten oxide layer.
 2. The anodic double layer gasochromic sensor structure of claim 1 wherein said palladium layer is 10 nm.
 3. The anodic double layer gasochromic sensor structure of claim 2 wherein said tungsten-doped nickel oxide layer and said palladium layer comprise anodic coloration materials.
 4. A method of preparing an improved gasochromic sensor for optical detection of hydrogen with improved response time and improved optical absorption real time constants, comprising: providing a substrate; depositing a tungsten-doped nickel oxide layer on said substrate; and depositing a palladium layer onto said tungsten-doped nickel oxide layer.
 5. The method of claim 4 wherein said tungsten-doped nickel oxide layer is deposited by reactive sputtering.
 6. The method of claim 5 wherein said absorption change is within a real time constant of about 10 seconds.
 7. The method of claim 6 wherein said palladium layer is evaporated onto said tungsten-doped nickel oxide layer.
 8. The method of claim 7 wherein said substrate is an optical fiber and said tungsten-doped nickel oxide on which said palladium layer is coated is placed onto said optical fiber.
 9. An improved method of optically detecting hydrogen with improved response time and improved optical absorption real time constants, comprising: subjecting an anodic double layer gasochromic sensor structure comprising a substrate, a layer of tungsten-doped nickel oxide coated on said substrate, and a layer of palladium coated on said tungsten-doped nickel oxide to an environment comprising hydrogen gas to cause a reaction of palladium and hydrogen to dissociate said hydrogen gas into H atoms on said palladium and to diffuse said H atoms into a W—NiO_(x) film to cause an anodic coloration in accordance with the equation: Ni³⁺Ni²⁺  and to cause an anodic coloration in accordance with the equation: PdPdH_(x).
 10. A method of optically detecting hydrogen gas with improved response time and improved optical adsorption real time constants that eliminates an ignition energy source at the leak site, thereby limiting the risk of explosion, comprising: subjecting an anodic double layer gasochromic sensor structure comprising a optical fiber, a layer of tungsten-doped nickel oxide coated on said optical fiber, and a layer of palladium coated on said tungsten-doped nickel oxide to an environment comprising hydrogen gas to cause a reaction of palladium and hydrogen to dissociate said hydrogen gas into H atoms on said palladium and to diffuse said H atoms into a W—NiO_(x) film to cause an anodic coloration in accordance with the equation: Ni³⁺Ni²⁺  said layer of tungsten-doped nickel oxide coated with palladium being placed within an optical fiber. 